1. Technical Field
The present invention relates to an electric disk brake which generates a brake force by pressing a brake pad to a disk rotor by use of an electric motor.
2. Description of the Related Art
As an electric disk brake, for example, Japanese Patent Public Disclosure 2003-202042 discloses an electric disk brake in which a rotational movement of a rotor of an electric motor is converted into a linear movement of a piston by use of a rotation-linear motion converting mechanism such as a ball screw mechanism or a ball ramp mechanism, and brake pads are pressed against a disk rotor by the piston, thereby generating a brake force. In the electric disk brake, a force of a driver's pressing a brake pedal (or displacement amount) is detected by a sensor, and rotation of the electric motor is controlled by a controller based on the detected value, whereby a desired brake force can be generated.
In this kind of electric disk brake, control accuracy can be improved by appropriate controlling of a brake force based on a thrust force of the piston or a rotational position of the electric motor. In addition, when the brake is not in operation, constant pad clearances can be maintained between the disk rotor and the brake pads based on the rotational position of the electric motor.
However, in the above-mentioned conventional electric disk brake, maintaining the constant pad clearances based on the rotational position of the electric motor may cause an undesirable effect as follows; for example, if runout of the disk rotor occurs due to, for instance, thermal deformation caused by a braking operation immediately after the end of the braking operation, the disk rotor may irregularly contact brake pads positioned in respective positions when the brake is not in operation. This contact occurs because small pad clearances are set to quickly perform a brake operation. At this time, in the above-mentioned electric disk brake, even though the disk rotor tries to retract the piston by contacting the brake pads, a position control is performed such that the electric motor is rotated so as to maintain the brake pads in the respective positions when the brake is not in operation. Consequently, the brake pads are pressed against the disk rotor, and the disk rotor tends to be unevenly abraded (or unevenly worn) to become ripply along a rotational direction. With the disk rotor unevenly abraded, brake judder is likely to occur.